To the 100 million plus North Americans addicted to their morning cup, this conversation may be irrelevant. Because of the one agreed upon negative effect: withdrawal. Caffeine dependency is a real thing. And we all know the effects of withdrawal are pretty nasty. Which means that all this science stuff -- especially the uncertainty -- isn't likely to change anyone's behavior. Same deal for the people who don't already drink coffee. For them, it's equally unlikely that the latest research will make a difference in their daily choice of beverage.

At the Same Time, I Know You're Curious

Whether it affects your coffee-drinking behavior or not, I know you still wanna know: Is coffee good for me? Bad for me? Or does a morning cup make no difference whatsoever to my health?

The most honest answer: I don't know. No one does. Not until they take a peek at your genes.

Coffee, Caffeine and Your Unique Liver

If you're not familiar with the field of nutrigenomics, prepare to be amazed.

It turns out that caffeinated coffee can either be a medicine or a poison. At least when it comes to one particular risk: heart disease. And it all hinges on one specific gene, called CYP1A2.

You see, the CYP1A2 gene tells your liver to make one of two enzymes: CYP1A2 fast or CYP1A2 slow. Now, here's where it gets interesting.

If you have the gene that makes the fast version of CYP1A2, each time you drink caffeinated coffee, your body processes and eliminates the caffeine from your bloodstream very quickly.

That effect -- being a fast metabolizer of caffeine -- leads to a decrease in heart disease risk when you drink a moderate amount (two to four cups) of caffeinated coffee each day.

(Stay thirsty my friends!)

However, if you have the gene that makes the slow version of CYP1A2, each time you drink caffeinated coffee, your body processes and eliminates the caffeine from your bloodstream very slowly.

That effect -- being a slow metabolizer of caffeine -- leads to an increase in heart disease risk even when drinking the same amount of caffeinated coffee each day.

(Damn these slow genes!)

Crazy, isn't it? When it comes to heart disease risk, caffeinated coffee can actually be a "poison" for one person. And a "medicine" for another. It all depends on one little gene.

Coffee, Caffeine and Your Unique Nervous System

Speaking of genes, here's another interesting story. Scientists have frequently observed that some people who drink caffeinated coffee have a lower risk for developing Parkinson's disease. However, not everyone sees this benefit. There are people who can drink lots of caffeinated coffee and still see no benefit in terms of Parkinson's risk.

What could possibly explain this? Well, nutrigenomics to the rescue again!

There's this gene called GRIN2A. I won't go all into the science but here's the bottom line: If your GRIN2A gene behaves one way, coffee is your best friend when it comes to Parkinson's risk, lowering it by almost 60 percent.

However, that doesn't necessarily mean coffee drinking is a good idea for me. Indeed, if I'm not careful, coffee still makes me jittery, jumpy and a bit neurotic. So, I don't drink it every day. In fact, I save it for the times where I really need an energy boost.

The rest of the time I start my day with a green tea and an anti-oxidant rich green food drink. This is the routine that feels best for me. But that's just me.

For everyone else -- especially you coffee drinkers who don't plan on doing the genetic testing thing -- it seems like your best bet is to limit intake to about one to three cups (eight to 24 ounces) of caffeinated coffee per day.

Then listen to your body. Sometimes it'll tell you everything you need to know.

(No genetic testing required.)

John Berardi, Ph.D., is a founder of Precision Nutrition, the world's largest online nutrition coaching company. In the last 5 years, Dr. Berardi and his team have personally helped over 20,000 people get healthier (and lose over 300,000 pounds of body fat) through their renowned coaching program, Lean Eating.